Offshore marine platforms resting on the bottom of the sea have not only to resist stresses due to swell but also stresses due to seismic shocks, this latter factor even being regarded as predominant in areas considered subject to strong seismic phenomena. However, the two types of disturbance proceeding from swell and from a seismic shock respectively, manifest themselves in frequency ranges far apart from each other. The result is that a structure which is designed to resist swell turns out to be too rigid to resist seismic shocks and that, on the contrary, a structure designed to resist seismic shocks is not sufficiently rigid to resist swell.
This has led, quite naturally, to the concept of relieving a disturbance reaction by putting into effect a so-called "controlled decoupling" system for platform structures. More precisely, it has been suggested that the structure be designed rigidly with regard to the action of swell, at the same time arranging integrating linkage parts of the said structure, which are designed to break following a seismic shock, in such a manner as to bring into play flexible interconnecting members held in reserve and arranged to back up the temporary integrating parts.